Fix doc error for Utf8Error

Author: ollie27

src/libcore/str/mod.rs

@@ -157,7 +157,7 @@ impl Utf8Error {
     /// // std::str::from_utf8 returns a Utf8Error
     /// let error = str::from_utf8(&sparkle_heart).unwrap_err();
     ///
-    /// // the first byte is invalid here
+    /// // the second byte is invalid here
     /// assert_eq!(1, error.valid_up_to());
     /// ```
     #[stable(feature = "utf8_error", since = "1.5.0")]